[TowerTalk] Vertical Pattern of a yagi
Ian White GM3SEK
gm3sek at ifwtech.co.uk
Tue Apr 25 03:53:19 EDT 2006
>EZNEC does display the pattern in free space, but it also gives the
>pattern over ground (of any characteristics) at any height, but only
>for flat ground. EZNEC cannot handle arbitrary terrains. For this you
>need HFTA. However HFTA cannot handle arbitrary antenna designs. HFTA
>works with single half wave elements or multiples thereof (2, 3, 4,
>etc... element yagis, or stacks).
>So you need both.
The most important factors in shaping your vertical pattern are the
antenna height, stacking (if you use it), and YOUR terrain profile.
Unless you live on flat terrain, you will need HFTA.
Compared with height, stacking and terrain, the free-space vertical
pattern of any particular yagi design has a relatively minor effect. Of
course there are many *other* important differences between individual
yagi designs; but in any practical installation, the shape of the yagi's
own free-space vertical pattern is almost completely overridden by the
much bigger shaping effects of height, stacking and terrain.
All that HFTA needs to include is a gradual narrowing of the free-space
vertical pattern with increasing gain, number of elements and boom
length. This needs nothing more than a limited menu of "generic"
antennas, with pre-defined "generic" vertical patterns. Simply pick the
one that best matches your number of elements and boom length.
If anyone is worried about the effects of the specific antenna design,
try running HFTA with the next-larger and the next-smaller generic
antennas. Alternatively, try running EZNEC with a range of yagi designs,
all at the same height over NEC's default flat terrain. In all cases,
you will find that the individual yagi design makes very little
difference to the vertical pattern, because the big picture is almost
completely dominated by height, stacking and terrain.
73 from Ian GM3SEK
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